The administration of veterinary medicines is very often accomplished by suspending such medicine in a viscous paste for oral administration to the animal. This has the advantage of providing for a highly concentrated formulation thus affording a small dose volume. This is particularly advantageous with medicines of low solubility where a drench, or orally administered aqueous solution, would necessitate a very large volume of solution. This would be particularly so for large animals. The viscous paste also has an advantage over a tablet or bolus which is orally administered because the paste is generally formulated to a tacky consistency so that when the paste is placed in the mouth of an animal, it sticks thereto so that the animal cannot expel the paste, which the animal generally can easily do in the case of a tablet or bolus. The normal mouth and tongue movements of the animal cause the paste to be slowly swallowed by the animal.
The administration of the veterinary paste may be accomplished with a piston operated paste dispenser. In one variation of such a dispenser, a large capacity reservoir is provided such that a large number of animals may be treated before the reservoir must be changed. The reservoir feeds the paste to a cylindrical piston operated pump, which provides the force to eject the paste into the mouth of the animal being treated.
The pump also generally provides some means of varying the size of the dose of veterinary paste such that animals of different sizes, weights, ages and physical conditions may be treated. It is most convenient to provide for a single full stroke of the piston which will deliver a dose adequate to treat the largest animal contemplated for treatment. Smaller animals may be treated by shortening the distance that the piston travels. However, in providing for such fractional doses, particularly when many fractional doses are provided sequentially, it has been observed that large air bubbles may form in the cylinder. The large air bubbles may originate as small air bubbles constrained within the paste, which in the confines of the cylinder, and during repeated fractional dosages accumulate to form larger and larger bubbles. These bubbles in the cylinder will not be ejected until the next full stroke of the piston. While these large air bubbles are trapped in the cylinder the dosage volume and accuracy are reduced, since the air bubbles not only take up part of the cylinder volume, but are also compressed during the ejection of the paste. As the piston travels to force the paste out of the cylinder, the force is used to compress the air bubble instead of providing an accurate dose of paste. The inaccurate dosages can severely effect the health of the animals being treated.
The instant invention provides for means to prevent the accumulation of such large air bubbles in the pump cylinder by inserting a conical spring within the cylinder. The conical spring is the same diameter as said cylinder and of about the same length as the piston travel during a full dosing stroke.